Suppression of human anti-porcine natural killer cell xenogeneic responses by combinations of monoclonal antibodies specific to CD2 and NKG2D and extracellular signal-regulated kinase kinase inhibitor

Natural killer (NK) cells can destroy xenogeneic tissues by antibody-dependent cell cytotoxicity (ADCC) and direct lysis. Unlike ADCC, activating interactions between human NK receptors and their cognate ligands in pigs are not fully elucidated. We set up this study to identify human NK activating receptors recognizing porcine cells isolated from distinct organs, e.g., aorta, cornea and liver, and to provide a molecular basis for effective immunosuppressive regimens. Among the array of NK receptors tested, NKp46, 2B4, CD49d, CD48, CD2 and NKG2D, only CD2 and NKG2D were shown to be involved in both cytotoxicity and cytokine (interferon-γ and tumour necrosis factor-α) production against porcine targets. Simultaneous blocking of CD2 and NKG2D by combining its monoclonal antibodies further suppressed xenogeneic NK responses. Moreover, addition of a suboptimal dose of PD98059, an extracellular signal-regulated kinase (ERK) kinase inhibitor, to those cells maximally reduced NK cytotoxicity, suggesting that ERK plays an important role in NK-mediated xenoreactivity. These impairments in NK cells were tightly associated with defective intracellular calcium mobilization and the subsequent degranulation process. Therefore, our data demonstrate a distinct role of CD2 and NKG2D on human NK cells in recognizing porcine grafts and further provide a potentially efficacious combinational regimen using anti-CD2 and anti-NKG2D monoclonal antibodies with PD98059 in a pig-to-human transplantation model.